Low RPMs

[rpickoff]

I have a 20' motorboat with a 4.3L 190 hp Merc with alpha one drive. The boat came with a 23" alum prop and I couldn't get RPMs above 3800. I switched to a 21" and now get to 4200. Suggested operating range is the high end of 4400-4800. Tach was checked with an independent tachometer. Any ideas what the issue could be?

[Thunderhead19]

Either your motor is not producing enough power or your prop pitch is to excessive.

[cyclops]

Are you able to get the boat on a full plane?

[rpickoff]

yes, pretty easily

[Jango]

Don't forget the Boat. Excessive weight or drag will pull down the RPMs, as well as motor tune and timing.

[marshmat]

rp, 23" pitch sounds kind of high for the 4.3. My grandfather's 18-foot Peterborough Gemini runs a 15"x17" aluminum 3-blade with the same motor; most I've seen are in the 16"-20" pitch range.

Could you give us as many of the following as possible:
Drive gear ratio, prop make, # of blades, diameter, pitch, boat weight, boat's beam and draught, boat's transom deadrise and hull shape.

A bit more information would make it easier to figure out what's up here.

[PAR]

I'm glad someone else noticed, I've got a 160 HP and Alpha spinning a 16/18

Typically on the side of the prop's center hub will be the manufacture and some numbers, some even the diameter and pitch.

23 or even 21 inches sounds to big to clear the plate on an Alpha. Measure from the center of the nut to the tip of any blade and multiply by 2 for the diameter.

[marshmat]

23", 21" is his pitch i think, my guess is it's about 14" dia. any more and he'd be lugging the motor like crazy. I seem to recall the Alpha's diameter limit is about 16-1/2 before you get clearance issues, not sure on that though.

[rpickoff]

Thanks for the reply. The boat weighs 2720, the gear ratio is 1.81, I think the draft is 19", ZP hull and I am not sure what the dead rise angle is.

[Jango]

With the no's given your speed is about 42 mph with either the 23 or 21 pitch. Not sure of your HP but with 190 HP at the flywheel, 3000#(total weight) and approx. 8% prop slip your speed should be about 48 mph, which equates to a 21 pitch, motor turning 4700 rpm. A 23 pitch is definitely too much prop unless you have a lot more HP.
I would suggest you check engine tune and excess weight in boat. Obviously Prop should be in good shape

[rpickoff]

[quote=Jango]With the no's given your speed is about 42 mph with either the 23 or 21 pitch. Not sure of your HP but with 190 HP at the flywheel, 3000#(total weight) and approx. 8% prop slip your speed should be about 48 mph, which equates to a 21 pitch, motor turning 4700 rpm. A 23 pitch is definitely too much prop unless you have a lot more HP.
I would suggest you check engine tune

Thank you very much for the info!

[rpickoff]

Thanks, yes it is the 190 hp 4.3L merc engine. I wonder why Stingray suggests the 23" prop to begin with!?!

[Jango]

Perhaps Stingray decided that 4400 - 4800 rpm was to high. With a 23 pitch, Engine rpm's would be 4200 -4300 @ 48 MPH

[marshmat]

Don't think that would be Stingray's business, that's Merc's business... doesn't stop stingray from meddling of course. Would try a 20-pitch, perhaps.

Advice: Make friends with your local prop shop owner. He'll have you test-drive a few wheels (take him in the boat with you) until you find something that's good. He'll want to sell you the right prop, that way you don't waste his time with returns, refunds and will be likely to go to him for repairs. It pays to test a few.

[Jango]

"Advice: Make friends with your local prop shop owner. He'll have you test-drive a few wheels (take him in the boat with you) until you find something that's good. He'll want to sell you the right prop, that way you don't waste his time with returns, refunds and will be likely to go to him for repairs. It pays to test a few."
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Keep in mind that 48mph is optimum.( Boat, Driver & 1/2 Tank fuel = 3000# )Each 200 lb shop owner will drop top end and rpm's about 2mph & 200rpm.

Jango